Ivens began his ministerial career at a time when the social gospel was on the rise in Canada. He was stationed at the McDougall Methodist Church in Winnipeg in 1916, and called for the church to lead the labour movement in its struggle against the prevailing tendencies of North American capitalism. He also supported the efforts of the Moral and Social Council of Canada to bring about large-scale social reform, and developed a reputation as a radical minister.

In 1917 and 1918, Ivens was opposed by several members of his congregation by defending conscientious objectors to World War I and criticizing the management of the war. He expressed these opinions as a private citizen in newspaper articles, rather than from his pulpit; nevertheless, many church members opposed him as insufficiently patriotic in wartime.

Although there were several petitions presented Ivens’s his favour (he had improved the financial status of McDougall during his time as its minister), the overseeing body removed him from the church in June 1918 in an effort to restore local unity. Ivens agreed to stand down on condition that he be granted a year’s leave to establish a “workers’ church”. Before the end of the month, he had founded the first Labour Church in the City of Winnipeg.

At the first meeting of the Labour Church on June 30, 1918, about two hundred attendees signed cards with the following declaration: “I am willing to support an independent and creedless Church based on the Fatherhood of God and the Brotherhood of Man. Its aim shall be the establishment of justice and righteousness among men of all nations.”

Ivens was directly involved in labour activism during this period. He supported the newly organized Dominion Labour Party, and attempted to build local networks of support in a speaking tour of western Canada. Upon his return to Winnipeg, he assumed the editorship of the Western Labour News, a newspaper published by the Winnipeg Trades and Labour Council. In 1919, he published a number of articles which supported the One Big Union movement.

Ivens continued as editor of the Western Labour News during the Winnipeg General Strike of 1919, and published a daily Special Strike Edition during this period. He wrote numerous articles in support of the strikers. Although he warned against public disorder, he also referred to the strike as the harbinger of a new age for the working-class in Canada. His Labour Church soon became the scene of extremely large meetings, with crowds of between 5,000 and 10,000 people emerging to support the strike effort.

When the Canadian government suppressed the strike in June 1919, Ivens was arrested on charges of seditious libel and conspiracy (J.S. Woodsworth took his place as editor). Although it is unlikely that his editorials actually constituted sedition by the standards of the age, he was found guilty in March 1920 and served time in jail. On Easter Sunday 1920, he led a prayer service for a crowd that had gathered around his prison cell.

While still in prison, Ivens ran as a candidate of the Dominion Labour Party in the provincial election of 1920, and was elected in the city of Winnipeg. Winnipeg, at the time, elected ten members by preferential balloting; Ivens finished fifth on the first count and was declared elected on the second after receiving transfers from DLP leader Fred Dixon. He entered the legislature following his release from prison in 1922.

In late 1920, the DLP split into two factions via a fallout over the Winnipeg General Strike. Dixon, Ivens, Woodsworth and most others on the left of the DLP founded the new Independent Labour Party.

In parliament, Ivens was known as an articulate and vociferous defender of labour interests though he was sometimes criticized for long-windedness in his speeches (Douglas L. Campbell once claimed that he heard Ivens speak for more than seven hours at a stretch). He was re-elected in the provincial election of 1922, but was reduced to fourth place among the successful social democratic candidates and was not assured of re-election until the final count.

In addition to his political career, Ivens also received a chiropractor‘s certificate in 1925 and practiced in the field.